Soldering a flexible circuit

ABSTRACT

Techniques are provided for controlling solder flow in applications where a flexible circuit is soldered to a microelectromechanical structure. A metal layer is formed on a substrate. A solder mask is formed on the metal layer such that portions of the metal layer are covered by the mask and portions are left exposed. A flexible circuit is soldered to the metal layer in at least some of the areas where the metal layer is exposed.

BACKGROUND

This invention relates to soldering flexible circuits to electricalcontacts on devices, such as printhead actuators.

Ink jet printers form an image by selectively depositing ink onto areceiving media. In a conventional ink jet printer system, the ink isstored in an ink storage unit, such as an ink reservoir or cartridge,and directed from the storage unit into a printhead 100, as shown inFIG. 1. In the printhead 100, ink flows into an ink pumping chamber 120to a nozzle 130, where the ink is ejected. Typically, the printheadincludes an actuator that forces ink out of the printhead 100 throughthe nozzle 130. Two common types of actuators include resistive heatingactuators and piezoelectric actuators. In a piezoelectric actuator 150,a layer of piezoelectric material 165 can be formed adjacent to the inkpumping chamber 120. Applying a voltage across the piezoelectricmaterial 165 causes the piezoelectric material to bend or deform, andthe deformation of the piezoelectric material 165 causes a pressure waveto propagate through the ink pumping chamber 120, pushing ink out of thenozzle 130 and onto the receiving media. Typically, electrodes 160, 170are formed on either side of the piezoelectric layer 165 to enablevoltage to be applied across the layer 165.

In so-called “drop on demand” printers, multiple flow paths 108 a and108 b (shown in phantom in FIG. 2) and associated nozzles 130 can beformed in a single printhead 100 and each nozzle 130 can be individuallyactivated. Thus, a particular nozzle fires only when a droplet of inkfrom that nozzle is desired. To activate a particular actuator on theprinthead, an electrical signal typically is individually communicatedto that actuator. The electrical signal can be communicated to theactuator by a flexible circuit connected to the printhead.

SUMMARY

In general, in one aspect, the invention features an actuator with firstand second electrodes and a piezoelectric layer disposed between theelectrodes. A mask is formed on the first electrode, wherein the mask isadjacent to a first portion of the first electrode. A solder material issupported by the first electrode and is adjacent to a second portion ofthe first electrode. The first portion of the first electrode does notoverlap the second portion of the first electrode and the mask includesa material that is substantially non-wettable by the solder materialwhen the solder material is melted.

The mask can include an oxide material. The mask can be between about0.1 and 2 microns thick, such as around 0.5 microns. The solder materialcan electrically connect the electrodes to an integrated circuit. Theactuator can be attached to a flexible circuit. The first portion cancorrespond to a location in a device when the second electrode is bondedto the device.

In another aspect, the invention features a method of forming amicroelectromechanical device. The method includes forming an actuatoron a top surface of a substrate, the actuator including a piezoelectriclayer, a first electrode and a second electrode. A solder mask is formedon the first electrode so that a first portion of the first electrode isexposed to the environment and a second portion of the first electrodeis covered by the solder mask. A solder is applied to the firstelectrode at the first portion of the first electrode. A flexiblecircuit is contacted to the solder. The solder is heated to cause thesolder to electrically connect the flexible circuit to the firstelectrode wherein the solder mask prevents the solder from flowing overthe first portion when the solder is heated.

Particular implementations can include one or more (or none) of thefollowing advantages. By forming the solder mask over the pumpingchamber of a printhead, the amount of solder that flows over the pumpingchambers can be reduced. A layer of solder on the actuator can cause theactuator to become very stiff and difficult to actuate. In addition, alayer of solder on the actuator can increase the mass of the actuator.Therefore, reducing the amount of solder over each pumping chamber canimprove the uniformity of the mass and flexural modulus of the actuatorfrom flow path to flow path and from printhead to printhead. This candirectly improve the uniformity of the actuator characteristics, such asthe drive characteristics. Thus, keeping solder from the active regionscan contribute to maintenance of uniform drive characteristics bothbetween flow paths and between printheads. A very thin layer of a soldermask, such as an oxide, can change the actuator characteristics verylittle. The mask material may tend to cause at least some types ofmelted solder to flow away from the oxide and toward a wettablematerial, such as a metal. Controlling the size and position of thesolder mask can be easier than controlling the melted solder without asolder mask. Any change in the actuators' performance caused by theaddition of the solder mask can be uniformly controlled. Controlling thesolder flow can also prevent electrical shorting of the printhead. Thedetails of one or more embodiments of the invention are set forth in theaccompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a side view of a single flow path in a printhead with apiezoelectric actuator.

FIG. 2 is a bottom view of a printhead with multiple nozzles.

FIG. 3 is a cross-sectional view of a printhead flow path with anactuator and circuit.

FIG. 4 is a cross-sectional view of a printhead flow path with apartially formed actuator.

FIGS. 5A and 5B are a cross-sectional views of a printhead with a soldermask.

FIG. 6 is a plan view of the membrane and actuator structures.

FIG. 7 is a cross-sectional view of a printhead with a solder mask andsolder.

FIG. 8 shows a printhead with a flexible circuit bonded to the actuator.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Techniques are provided for controlling the location of solder when thesolder is melted on an actuator. The techniques can be implemented tocontrol the flow of solder used to connect an integrated circuit to theactuator of a microelectromechanical device, such as an ink jetprinthead.

Referring to FIG. 3, a printhead 100 includes a substrate 105 in whichmultiple flow paths 108 are formed. A single flow path 108 can includean ink inlet 142, an ascender 135, a pumping chamber 120, a descender138 and a nozzle 130. A piezoelectric actuator 150 is supported by thesubstrate 105. The actuator 150 can include a membrane 140 that sealsthe pumping chamber 120. The actuator 150 can include a lower electrode160, a piezoelectric layer 165 and an upper electrode 170. Theelectrodes 160, 170 can be about two microns in thickness or less, suchas about 0.5 microns. The piezoelectric layer 165 can be between about 1and 25 microns thick, e.g., about 8 to about 18 microns thick. Theelectrodes 160, 170 are formed of a conductive material, such as ametal, e.g., copper, gold, tungsten, tin, indium-tin-oxide (ITO),titanium, platinum, nickel, nickel chromium alloy or a combination ofmetals. A signal can be provided to the electrodes 160, 170 toelectrically activate the actuator 150. An electrode separation gap 172can separate the upper electrode 170 from a lower electrode 160.

Kerfs 176 separate individual actuators and allow for connecting theelectrodes. A first kerf (not shown) can separate the actuator over oneflow path from the actuator over a neighboring flow path. A second kerf176 in the actuator 150 can separate neighboring actuators. In addition,the second kerf 176 can reduce the actuator size such that the actuatoris only over a portion of each corresponding flow path. The kerfs canreduce crosstalk between the actuators.

Referring to FIG. 4, an actuator is formed on a substrate 105 with theflow path features formed therein. The actuator can be formed by anysuitable method. One particular method is described below. Initially,the piezoelectric layer 165 and the lower electrode 160 can be appliedto the back side of the substrate 105. In one implementation, thepiezoelectric layer 165 is metalized with a metal that will subsequentlyform the lower electrode 160. The piezoelectric layer 165 can be formedof a ceramic green sheet or a prefired piezoelectric material. The metalcan be deposited by sputtering. The metals for deposit can includecopper, gold, tungsten, tin, indium-tin-oxide (ITO), titanium, platinum,nickel, nickel chromium alloy or a combination of two or more of thesemetals. The piezoelectric layer 165 is then bonded onto the substrate,such as with an adhesive or with a eutectic bond between two metals. Inanother implementation, the substrate 105 is metallized and thepiezoelectric layer 165 is formed on the metal layer, such as byphysical vapor deposition (PVD), sol gel application, bonding ceramicgreen sheets or another suitable deposition process.

Kerfs 176 are then formed in the piezoelectric layer 165. The kerfs 176can be cut, diced, sawed or etched into the piezoelectric layer 165. Thekerfs 176 can extend into the lower electrode 160 as well as thepiezoelectric layer 165. The piezoelectric layer 165 can be metalized,such as by vacuum depositing, e.g., sputtering, to form the upperelectrode 170, the lower electrode contact area 162 and a via 123 on thepiezoelectric layer 165. The top metallization can be patterned toremove metal in the kerf 176 and in an electrode separation gap 172.

Referring to FIG. 5A, the printhead 100 is shown with a solder mask 195.The solder mask 195 is formed on the upper electrode 170 over a firstregion 194 of the upper electrode 170 that overlies the pumping chamber120. The edge of the solder mask 195 can extend to the edge of the firstregion 194 or extend beyond the first region 194. From one flow path tothe next flow path, the extent of the solder mask 195 is kept uniform toimprove uniformity between the flow paths. The solder mask 195 can havea thickness between about 0.1 and 5 microns, such as about 0.5 microns.The solder mask can be configured to cover more or less of the upperelectrode 170. In one implementation, the solder mask 195 is configuredto control the location of the solder on the lower electrode contactarea 162, as shown in FIG. 5B.

To form the solder mask 195, a layer of the material used to make themask is deposited on the upper electrode 170, such as by a plasmaenhanced chemical vapor deposition technique. The solder mask 195 can beformed of an oxide, such as a silicon oxide. A photopatternable materialor photoresist is applied on the surface of the solder mask material. Amask is provided over the photoresist that corresponds to the regions194 over the pumping chamber 120. The photoresist is exposed anddeveloped. The solder mask material is etched, such as by a dry etchprocess, in the areas no longer covered by the photoresist. Inductivelycoupled plasma reactive ion etching is one example of an etch processthat can be used to etch the exposed portions of the solder maskmaterial. After the solder mask material is etched, the remainingmaterial is substantially confined to the region 194 overlying thepumping chamber 120. The remaining photoresist is then removed from theupper electrode 170.

Referring to FIG. 6, a plan view of a portion of the membrane 140, withthe upper electrode 170, exposed piezoelectric layer 165, lowerelectrode contact area 162, solder mask 195 and kerf 176.

Referring to FIG. 7, a solder 190 is applied to the upper electrode 170.The solder 190 can be forced through a mask onto the substrate. Solder190 is applied to the upper electrode 170 to form an electrical contactfor the upper electrode 170. Solder 192 is applied in the lowerelectrode contact 162 area to form an electrical contact to the lowerelectrode 160. The solder includes a conductive material, such as ametal, including tin and lead, that can be heated to a temperature thatcauses the metal to flow and form a electrical bond to anotherconductive material, such as the upper and lower electrodes 170, 160.

Referring to FIG. 8, an integrated circuit, such as an integratedcircuit that is attached to a flexible circuit 180, is electricallyconnected to the upper and lower electrodes 170, 160. The flexiblecircuit can include contact pads that are electrically connected to theintegrated circuit. The contact pads allow the flexible circuit to beelectrically connected to the upper and lower electrodes 170, 160. Inone implementation, the upper electrode contact pads provide the drivevoltage while the lower electrode pads are electrically connected toground.

The flexible circuit 180 and substrate 105 are run through a thermalcycle, such as around 183° C., causing the solder to flow. The meltedsolder forms a bond to both the electrodes and the contact pads. Theelectrodes are therefore conductively connected to the integratedcircuits through the contact pads. The solder mask 195 prevents themelted solder from flowing over the mask 195 because the solder mask 195is not wetted by the solder 190. When the solder returns to atemperature below that at which the solder flows, the solder returns toa solid form.

Forming a solder mask onto the actuator prior to bonding the flexiblecircuit can be advantageous in that the mask can control the location ofthe solder when the solder is melted. Without the oxide layer, thesolder can spread in an uncontrolled manner over the actuator. In someportions of the actuator, kerfs are cut to electrically isolate layers,such as the upper electrode and the lower electrode. In oneimplementation, the solder mask can be applied in the kerfs 176. If thesolder flows to areas where the solder connects layers that should beelectrically isolated, electrical shorting can occur. A solder mask inthese areas can prevent shorting between the electrodes, such as in theelectrode separation gap 172.

The solder mask can be formed over the region of the actuator over thepumping chamber and any other region where solder is not desired. Anoxide, such as a silicon oxide, can be selected for the solder maskbecause oxides are not wettable by a melted solder. Oxides tend to bestable and can be formed in a very thin layer while still retainingnon-wettable characteristics. However, other materials, such as nitride,polyimide or other patternable materials, can alternatively be used toform the solder mask.

As mentioned above, applying a solder mask prevents the solder fromcovering the area of the actuator that overlies the pumping chamber.This maintains uniform solder application from one actuator to the nextactuator. Uniform solder application can help maintain uniform actuatorcharacteristics. The mass of the portions of the actuator that overliethe pumping chamber can be kept down to little more than the mass of theactuator components, i.e., the electrodes, the piezoelectric layer andthe membrane. Any additional mass from the oxide layer can becontrolled. Conversely, additional mass from solder is more difficult tocontrol. Adding mass to the active region is undesirable because itchanges the drive characteristics of the associated actuator. Inaddition, adding the solder layer to the actuator increases thestiffness of the actuator, making the actuator more difficult to bend.The combination of forming substantially consistently sized solder masksto each actuator and preventing solder from flowing over the activeregions contributes to uniformity between actuators of a printhead orbetween printheads.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, the piezoelectric actuator can form a side wall of the pumpingchamber. The solder mask can be applied to an actuator that has anelectrode only on one side of the piezoelectric layer, rather than onboth sides. The invention can also be applied to ejecting fluids otherthan ink from microelectromechanical structures. Alternatively, theinvention can be applied to any sensor microelectromechanical structurethat requires bonding a connection to an integrated circuit. Thetechniques disclosed above can be used to control the placement ofsolder when soldering any components on a microelectromechanical device.Accordingly, other embodiments are within the scope of the followingclaims.

1. An actuator, comprising: a first electrode; a support substrate forthe first electrode; a mask formed on the first electrode, wherein themask is adjacent to a first portion of the first electrode; and a soldermaterial, wherein the solder material is supported by the firstelectrode and adjacent to a second portion of the first electrode,wherein the first portion of the first electrode does not overlap thesecond portion of the first electrode and the mask includes a materialthat is substantially non-wettable by the solder material when melted.2. The actuator of claim 1, wherein: the mask includes an oxidematerial.
 3. The actuator of claim 1, further comprising: a secondelectrode, wherein the solder material electrically connects the firstand second electrodes to an integrated circuit.
 4. The actuator of claim1, further comprising: a flexible circuit, wherein the solder materialelectrically connects the flexible circuit to the first electrode. 5.The actuator of claim 1, wherein: the first portion corresponds to alocation of a chamber in a device when the second electrode is bonded tothe device.
 6. The actuator of claim 1, wherein: the mask is betweenabout 0.1 and about 2 microns thick.
 7. The actuator of claim 6,wherein: the mask is about 0.5 microns thick.
 8. The actuator of claim1, wherein: the support substrate includes a piezoelectric layer.
 9. Aprinthead, comprising: a substrate in which flow path features areformed, the flow path features including a pumping chamber and a nozzle;and an actuator bonded to the substrate, the actuator comprising: apiezoelectric layer; a first electrode supported by the piezoelectriclayer; a mask formed on the first electrode, wherein the mask isadjacent to a first portion of the first electrode and the first portionof the first electrode substantially overlies the pumping chamber; and asolder material, wherein the solder material is supported by the firstelectrode and adjacent to a second portion of the first electrode,wherein the first portion of the first electrode does not overlap thesecond portion of the first electrode and the mask includes a materialthat is substantially non-wettable by the solder material when thesolder material is melted.
 10. The printhead of claim 9, furthercomprising: a second electrode, wherein the substrate is closer to thesecond electrode than the first electrode.
 11. The printhead of claim 9,wherein: the mask is between about 0.1 and about 2 microns thick. 12.The printhead of claim 11, wherein: the mask is about 0.5 microns thick.13. The printhead of claim 9, wherein: the mask includes an oxide.
 14. Amethod of forming a microelectromechanical device, comprising: formingan actuator on a top surface of a substrate, the actuator including apiezoelectric layer and a first electrode; forming a solder mask on thefirst electrode so that a first portion of the first electrode isexposed to the environment and a second portion of the first electrodeis covered by the solder mask; applying solder to the first electrode atthe first portion of the first electrode; contacting a flexible circuitto the solder; and heating the solder to cause the solder toelectrically connect the flexible circuit to the first electrode whereinthe solder mask prevents the solder from flowing over the first portionwhen the solder is heated.
 15. The method of claim 14, wherein: forminga solder mask includes applying a material that is substantially notwettable by the solder when melted.
 16. The method of claim 14, wherein:forming a solder mask includes depositing an oxide.
 17. The method ofclaim 16, wherein: providing the substrate, wherein the substrateincludes a pumping chamber; and forming a solder mask includes forming asolder mask that does not melt at or below a melting temperature of thesolder and is located substantially over the pumping chamber so thatwhen the solder is heated the solder mask prevents the solder fromflowing over the pumping chamber.
 18. The method of claim 17, wherein:forming the solder mask includes forming the solder mask includesforming the solder mask to be between about 0.1 and about 2 micronsthick.
 19. The method of claim 18, wherein: forming the solder maskincludes forming the solder mask to be about 0.5 microns thick.